Image forming process by diffusion transfer is well known. Specifically, this technique comprises processing an imagewise exposed light-sensitive silver halide emulsion layer with an alkaline aqueous solution containing a developing agent and a silver halide solvent, to reduce the exposed silver halide grains to silver with the developing agent and convert the unexposed silver halide grains into a diffusible silver complex salt with the silver halide solvent, and transferring the silver complex salt to a silver precipitating agent-containing layer (image receiving layer) disposed in superimposition with said emulsion layer by way of imbibition, where the silver complex salt is reduced by the developing agent with the aid of the silver precipitating agent to provide a silver image. This technique is generally carried into practice by using a film unit which comprises a light-sensitive element having a light-sensitive silver halide emulsion layer on a support, an image receiving element having an image receiving layer containing a silver precipitating agent on a support, and a processing element comprising a rupturable container containing a viscous alkaline aqueous solution containing a developing agent, a silver halide solvent and a thickener. To begin with, the emulsion layer of the light-sensitive element is imagewise exposed and the light-sensitive element and image receiving element are laminated in such a manner that said emulsion layer faces said image receiving layer, and are passed between a pair of rollers so as to destroy said processing element and thereby spread said viscous alkaline aqueous solution therebetween, and, after the laminate is allowed to stand for a given time, the image receiving element is peeled apart from the light-sensitive element to provide a desired image formation on the image receiving layer.
In regard to the image receiving element for silver salt diffusion transfer photography, Japanese Patent Publication No. 32754/69 (corresponding to U.S. Pat. No. 3,671,241) discloses an image receiving material prepared by incorporating a silver precipitating agent in an alkaline-impermeable polymer by vacuum deposition technique, dissolving the same in a solvent for said polymer, coating the solution onto a support, and, after drying, treating the surface layer of the polymer layer by chemical means such as hydrolysis to render the surface alkali-permeable. Japanese Patent Publication No. 49411/76 describes a production method in which a silver precipitating agent is embedded during saponification of a cellulose ester layer or thereafter. This method provides an image receiving layer having high mechanical strength characteristics. U.S. Pat. No. 3,671,241 teaches a method of preparing an image receiving layer which comprises saponifying a cellulose ester layer in which a silver precipitating agent has been incorporated.
As regards the developing agent for silver halide grains, the silver halide developing agents of hydroxylamine type have been found to be especially valuable, for when this type of developing agent is employed in association with a silver image receiving layer of regenerated cellulose type, a silver transfer image can be obtained which does not require, at all or substantially, an aftertreatment. Particularly useful silver halide developing agents of hydroxylamine type are N-alkyl and N-alkoxyalkyl-substituted hydroxylamines. A number of such hydroxylamine compounds have been described in U.S. Pat. Nos. 2,857,274, 2,857,275, 2,857,276, 3,287,124, 3,287,125, 3,293,034, 3,362,961, and 3,740,221. Particularly effective and desirable hydroxylamines are those having the formula ##STR1## wherein R.sup.1A is an alkyl, alkoxyalkyl, or alkoxyalkoxyalkyl group, and R.sup.2A is a hydrogen atom or an alkyl, alkoxyalkyl, alkoxyalkoxyalkyl, or alkenyl group. Preferably, said alkyl, alkoxy, and alkenyl groups each contains from 1 to 3 carbon atoms. Among especially useful silver halide developing agents of the hydroxylamine type are N,N-diethylhydroxylamine, N,N-bis-methoxyethylhydroxylamine, and N,N-bisethoxyethylhydroxylamine.
Moreover, said developing agent may be used in combination with auxiliary developing agents, such as phenidone compounds, p-aminophenol compounds, and ascorbic acid.
As stated previously, the diffusion transfer system using a regenerated cellulose image receiving layer has the disadvantage of a long image formation time as compared with the system employing a colloidal silica image receiving layer, as described in U.S. Pat. No. 3,671,241.
Furthermore, it is mentioned in Japanese Patent Publication No. 29417/80 that when a regenerated cellulose layer containing a silver precipitating agent is used as the image receiving layer, the quality of the silver transfer image can be improved by using a silver halide photographic emulsion consisting substantially completely of silver halide grains layer than 0.5 .mu.m in diameter.
However, it has been found that with silver halide photographic emulsions according to the above-mentioned patent literature, the image formation time is undesirably long and the maximum density of the transfer image is undesirably low.